Sheet detector, sheet detector mechanism and image forming apparatus

ABSTRACT

A sheet detector, a sheet detector mechanism and an image forming apparatus. An image forming apparatus may include an image forming mechanism, a sheet conveyance mechanism, and a sheet detector. The image forming mechanism may form an image and transfer the image onto the sheet conveyed by the sheet conveyance mechanism. The sheet detector may be arranged upstream from an image transfer region and configured to detect the sheet. The sheet detector may include a light source emitting light, a plurality of photoreceptors, and a controller. The plurality of photoreceptors may be arranged in a line in a main-scanning direction and may include a first photoreceptor group and a second photoreceptor group. The first photoreceptor group may change output voltages when overridden by the side edge of the sheet. The second photoreceptor group may include an opposite end of the plurality of photoreceptors being out of the side edge of the sheet. The controller, which may be part of the sheet detector mechanism, may be configured to adjust a light emission amount of the light source in accordance with characteristics of the sheet.

BACKGROUND

1. Field

Example embodiments generally relate to a sheet detector, a sheetdetector mechanism and an image forming apparatus, for example, to animage forming apparatus including a sheet detector capable of detectinga position of a sheet.

2. Discussion of the Background

In general, a background image forming apparatus, for example, a copyingmachine, a printer, a facsimile machine, and a so-called multifunctionprinter, includes an image forming mechanism for forming an image, e.g.,a toner image, and a sheet conveyance unit for conveying a recordingsheet, e.g., a paper sheet, to the image forming mechanism.

These machines have recently been provided with a versatility ofhandling a variety of differently-sized recording sheets in order meet amarket demand. For example, background image forming apparatus may beequipped with a plurality of sheet containers for containing variouskinds and/or sizes of recording sheets.

Such enhancement of the recording sheet handling, however, may increasea number of sheet conveyance paths provided in the sheet conveyanceunit, each to reach a common region in which an image is transferredfrom the image forming mechanism to the recording sheet. If the sheetconveyance paths have greater tolerances relative to the common region,a position for an image transferred on a recording sheet may inevitablyvary depending upon the kinds and/or sizes of the recording sheets.

If an image transfer positioning relative to the recording sheetsconveyed by is not accurate, the recording sheets may have unevenmargins or a partly-cut-off image. Further, if a stack of recordingsheets having such uneven margins is subjected to a book-binding-likeprocess, a part of the images may be cut off.

SUMMARY

In example embodiments, an image forming apparatus may include an imageforming mechanism, a sheet conveyance mechanism, and/or a sheetdetector. The sheet conveyance mechanism may be configured to convey asheet through a sheet conveyance passage. The image forming mechanismmay be configured to form an image and to transfer the image at an imagetransfer region onto the sheet conveyed by the sheet conveyancemechanism. The sheet detector may be arranged upstream from an imagetransfer region in a sheet conveyance direction and configured to detectthe sheet. The sheet detector may include a light source for emittinglight, a plurality of photoreceptors, and/or a controller. The pluralityof photoreceptors may be arranged in a line in a main-scanning directionwith space at substantially equal intervals to extend across a sheetskew marginal width and to evenly receive the light from the lightsource. The plurality of photoreceptors may include a firstphotoreceptor group and a second photoreceptor group. The firstphotoreceptor group may be configured to be overridden by a side edge ofthe sheet passing through the sheet conveyance passage and to changeoutput voltages when overridden by the side edge of the sheet. Thesecond photoreceptor group is connected to the first photoreceptor groupand includes an opposite end of the plurality of photoreceptors beingout of the side edge of the sheet. The controller, which may be part ofa sheet detector mechanism, may be configured to adjust a light emissionamount of the light source in accordance with characteristics of thesheet to maintain the output voltages of the plurality ofphotoreceptors, regardless of the characteristics of the sheet.

In example embodiments, a sheet detector mechanism may include a sheetdetector including a light source emitting light and a plurality ofphotoreceptors arranged in a line in a main-scanning direction spaced atsubstantially equal intervals to extend across a sheet skew marginalwidth and to evenly receive light from a light source, the plurality ofphotoreceptors including a first photoreceptor group overridable by aside edge of the sheet passing through a sheet conveyance passage and asecond photoreceptor group connected to the first photoreceptor groupand including an opposite end of the plurality of photoreceptors beingout of the side edge of the sheet, the first photoreceptor groupchanging output voltages when overridden by the side edge of the sheet;and a controller configured to adjust a light emission amount of thelight source in accordance with characteristics of the sheet so as tomaintain the output voltages of the plurality of photoreceptors,regardless of the characteristics of the sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of an image forming apparatus according toexample embodiments;

FIG. 2 is an illustration to explain a transmission sheet detectoraccording to example embodiments;

FIG. 3 is an example illustration to explain output voltages of thesheet detector of FIG. 3;

FIG. 4 is an illustration to explain a reflection sheet detectoraccording to example embodiments;

FIG. 5 is an example illustration to explain a light emission part and aphotoreceptor part included in the sheet detector of FIG. 4;

FIG. 6 is an example illustration to explain output voltages of thesheet detector of FIG. 4;

FIG. 7 is an illustration of an example operation part;

FIG. 8 is an illustration of another example of operation part;

FIG. 9 is a flowchart of an example of a detection condition settingprocedure; and

FIG. 10 is a flowchart of another example of a detection conditionsetting procedure.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In describing example embodiments illustrated in the drawings, specificterminology is employed for the sake of clarity. However, the disclosureof this patent specification is not intended to be limited to thespecific terminology so selected and it is to be understood that eachspecific element includes all technical equivalents that operate in asimilar manner. Referring now to the drawings, wherein like referencenumerals designate identical or corresponding parts throughout theseveral views, particularly to FIG. 1, an image forming apparatus 100according to example embodiments is described.

The image forming apparatus 100 may be a copier, a printer, a facsimilemachine, or a multifunction peripheral (MFP) having at least two of theabove functions. As illustrated in FIG. 1, the image forming apparatus100 may include an image forming mechanism 1, a sheet conveyance unit10, a pair of registration rollers 11, a sheet detector 14, and/of anoperation part 25. The sheet detector 14 may be provided upstream of theimage forming mechanism 1 in a sheet conveyance direction and may detecta position of a recording sheet transported by the sheet conveyance unit10. The sheet conveyance unit 10 may include a sheet feeder 21 that maybe provided beneath the image forming mechanism 1.

The image forming mechanism 1 may form an image on a recording sheet andmay include an image carrier 2, a charging device 3, an exposure device4, a transfer unit 5, a cleaning device 6, a fixing unit 7, and/or adeveloping unit 8. The sheet feeder 21 may a supply sheet P as arecording sheet and include a feeding roller 22 to forward the sheet Pand a sheet cassette 23 to store a plurality of sheets P. The transferunit 5 may include a transfer belt 12 and a transfer roller 13. Thetransfer belt 12 may be stretched around a plurality of rollers androtate in a direction of arrow B.

The image carrier 2 may be a drum-shaped photoconductor. In an imageforming process, the image carrier 2 may rotate clockwise in FIG. 1. Thecharging device 3 may charge a surface of the image carrier 2 with apredetermined or given polarity. The exposure device 4 may irradiate thecharged surface with a laser beam L to form an electrostatic latentimage. The laser beam L may be optically modulated. The developing unit8 may develop the electrostatic latent image with a toner into a tonerimage.

In the sheet feeder 21, the feeding roller 22 may be in contact with asheet P that is on the top in the sheet cassette 23. The sheets P may besent out from the top when the feeding roller 22 rotates. The sheetconveyance unit 10 may transport the sheet P in the direction of arrow Ato the pair of registration rollers 11. The sheet P may be stopped whenits front edge is sandwiched between the registration rollers 11. Theregistration roller 11 may rotate at a predetermined or desirable timingto send the sheet P to a space between the image carrier 2 and thetransfer belt 12.

The transfer belt 12 may be charged by the transfer roller 13. While thesheet P is conveyed by the transfer belt 12, the toner image on theimage carrier 2 may be transferred onto the sheet P by the effect of thetransfer voltage. The cleaning device 6 may remove a toner remains onthe image carrier 2 after the transfer process. The fixing unit 7 mayfix the toner image on the sheet P.

In addition to the sheet feeder 21 illustrated in FIG. 1, a manual sheetfeeder to forward sheets that are manually put therein by an operatorand/or a large capacity sheet feeder may be provided.

The operation part 25 may be provided on a top of the image formingapparatus 100 for an operator to select functions and/or to inputsettings.

The sheet detector 14 may detect a position of the sheet P in thedirection perpendicular to the sheet conveyance direction, in otherwords, in the width direction of the sheet P, to enhance positionalaccuracy of an image relative to the sheet P. In FIG. 1, the sheetdetector 14 may be provided upstream of a transfer region, where thetoner image on the image carrier 2 is transferred onto the sheet P inthe sheet conveyance direction as an example.

Based on the result of detection by the sheet detector 14, a startingpoint for the exposure device 4 to irradiate the image carrier 2 in themain-scanning direction may be corrected. An electrostatic latent imageformed on the image carrier 2 may be developed as a toner image by thedeveloping unit 8. The toner image may be transferred onto the sheet P.Thus, the position of image relative to the sheet P may be correctedbased on the result of detection by the sheet detector 14.

The sheet detector 14 is described in detail with reference to FIG. 2.The sheet detector 14 may be a transmission detector and include a lightemission part 15, a photoreceptor part 16, and/or a controller 40. Thesheet detector 14 may be connected to the operation part 25. The lightemission part 15 may include a light emission element 17 as a lightsource, and a light guide 18.

The photoreceptor part 16 may include a photoreceptor row 19 in which aplurality of photoreceptor elements 20 is arranged in line in amain-scanning direction that is perpendicular to the sheet conveyancedirection at substantially constant intervals to extend across a sheetskew marginal width. Each of the photoreceptor elements 20 may beconfigured to evenly receive light from the light emission element 17.The photoreceptor row 19 may include photoreceptor element 20 g providedat one end thereof (first end) and a photoreceptor element 20 h at theother end thereof (second end). In FIG. 2, the photoreceptor element row19 may be divided in photoreceptor element groups 20 a and 20 b.Although the light emission part 15 may include one light emissionelement 17 as an example, the light emission part 15 may include aplurality of light emission elements.

FIG. 2 illustrates a situation in which the sheet P reaches the sheetdetector 14 and a width direction of the sheet P is shown by arrow w.The sheet P may be conveyed in a vertical direction relative to asurface of FIG. 2. The sheet P may include a first surface s1, a secondsurface s2, and an edge E.

The light emission part 15 may face the first surface s1 and thephotoreceptor part 16 may face the second surface s2 of the sheet P.

In FIG. 2, the photoreceptor element group 20 a may face the sheet P andthe photoreceptor element group 20 b does not face the sheet P. Thephotoreceptor element 20 h may be located at a position not facing thesheet P and the photoreceptor element 20 g may be located at a positionfacing the sheet P, whatever size the width of the sheet P is.

The light emission element 17 may emit light when the sheet P reachesthe sheet detector 14. The light may be guided to the plurality ofphotoreceptor elements 20 by the light guide 18 as shown by arrows. Theplurality of photoreceptor elements 20 may output different voltagescorresponding to the amount of light received (output voltage) to thecontroller 40. The photoreceptor element group 20 b may receive a largeramount of light than the amount of light received by the photoreceptorelement group 20 a. Therefore, the position of the edge E of the sheet Pmay be determined based on the output voltages from the plurality ofphotoreceptor elements 20 corresponding to the amount of light received.

FIG. 3 is an illustration to explain output voltages from the pluralityof photoreceptor elements 20 included in the sheet detector 14illustrated in FIG. 2. In FIG. 3, the photoreceptor element row 19 isillustrated along a horizontal axis and a vertical axis illustratesoutput voltages from the plurality of photoreceptor elements 20. Thephotoreceptor element row 19 may include photoreceptor element groups 20a, 20 b, 20 c, and 20 d.

The output voltages from the plurality of photoreceptor elements 20 weremeasured when positions in width direction of sheets P having differentthickness were detected by the sheet detector 14. Each of the lines P1,P2, and P3 indicates the output voltages from the plurality ofphotoreceptor elements 20 when the sheet P was a normal paper (P1), athin paper (P2), or a cardboard (P3).

As illustrated in FIG. 3, the output voltages from the photoreceptorelement group 20 a facing the sheet P is smaller than the outputvoltages from the photoreceptor element group 20 b not facing the sheetP. Therefore, the position of the edge E of the sheet P may be detectedby preliminary setting a threshold Fth of output voltage from thephotoreceptor elements 20. In FIG. 3, the threshold Fth may be set whenthe sheet P is the normal paper.

The controller 40 may set a detection condition and determine whether ornot the output voltage is equal to or less than the threshold Fth whenthe light emission element 17 emits light. The controller 40 maydetermine that the sheet P exists at the position facing thephotoreceptor element 20 in the photoreceptor element group 20 a whoseoutput voltage is equal to or less than the threshold Fth. Therefore,the controller 40 may detect the position of the sheet P in widthdirection and determine the position of its edge E.

However, the amounts of light received by the photoreceptor elements 20may differ depending on differences in characteristics, for example,thickness, of the sheet P. The thin paper may transmit more light thanthe normal paper, and the cardboard may transmit less light than thenormal paper. Accordingly, the output voltages from the photoreceptorelements 20 may differ. For example, the output voltage from thephotoreceptor group 20 c is smaller than the threshold Fth when thesheet is the thin paper, as illustrated in FIG. 3. Similarly, the outputvoltage from the photoreceptor group 20 d may be smaller than thethreshold Fth when the sheet is the thick cardboard. The above problemmay be solved by adjusting the amount of light emission.

The sheet detector according to example embodiments may be a reflectiondetector. FIG. 4 illustrates a sheet detector 14 a that is a reflectiondetector. The sheet detector 14 a may include a light emission part 15 aand a photoreceptor part 16. The light emission part 15 a may include atleast one light emission element 17. The photoreceptor part 16 mayinclude a photoreceptor element row 19. In the sheet detector 14 a, thelight emission part 15 a and the photoreceptor part 16 may be providedat a same side of the sheet P (side of surface s1 in FIG. 4), unlike thesheet detector 14 illustrated in FIG. 2.

FIG. 4 illustrates a situation in which the sheet P is conveyed in thedirection of arrow A and reaches the sheet detector 14 a. When the sheetP reaches the sheet detector 14 a, the light emission element 17 mayemit light to the sheet P. The photoreceptor element row 19 may receivethe light reflected by the sheet P.

FIG. 5 illustrates the light emission part 15 a and photoreceptor part16 of the sheet detector 14 a in detail. The photoreceptor element row19 may include a plurality of photoconductor elements 20 lined at asubstantially constant intervals in a main-scanning direction that isperpendicular to the sheet conveyance direction shown by arrow A. Thephotoreceptor element row 19 may include a photoreceptor element group20 a facing the sheet P and the photoreceptor element group 20 b notfacing the sheet P. The photoreceptor element groups 20 a and 20 b mayinclude photoreceptor elements 20 h and 20 g, respectively. Thephotoreceptor element 20 h may be located at a position not facing thesheet P and the photoreceptor element 20 g is located at a positionfacing the sheet P, whatever size the width of the sheet P is. Thephotoreceptor element group 20 a may receive a larger amount of lightthan the amount of light received by the photoreceptor element group 20b, contrary to the sheet detector 14 illustrated in FIG. 2.

FIG. 6 is an illustration to explain output voltages from the pluralityof photoreceptor elements 20 included in the sheet detector 14 aillustrated in FIG. 4. In FIG. 6, the photoreceptor element row 19 isillustrated along a horizontal axis and a vertical axis illustratesoutput voltages from the plurality of photoreceptor elements 20. Thephotoreceptor element row 19 may include photoreceptor element groups 20a, 20 b, 20 e, and 20 f.

The output voltages from the plurality of photoreceptor elements 20 weremeasured when sheets P having different reflectivity were conveyed tothe sheet detector 14 a and the light emission part 15 a emits light.Each of the lines P4, P5, and P6 indicates the output voltages when thesheet P was a normal paper (P4), a light-colored paper (P5), or adark-colored paper (P6). The light-colored paper is likely to reflectmore light than the amount of light reflected by the normal paper. Thedark-colored paper is likely to reflect less light than the amount oflight reflected by the normal paper.

The output voltages of light received by the photoreceptor element group20 a facing the sheet P are larger than the output voltages from thephotoreceptor element group 20 b not facing the sheet P, as illustratedin FIG. 6. Therefore, the position of the edge E of the sheet P may bedetected by preliminary setting a threshold value Fth of output voltage.

In FIG. 6, the threshold Fth is set when the sheet P is the normalpaper. When the light emission element 17 emits light, the controller 40may determine that the sheet P exists at the position facing thephotoreceptor element 20 in the photoreceptor element group 20 a whoseoutput voltage is equal to or greater than the threshold Fth. Therefore,the controller 40 may detect the position of the sheet P in widthdirection and determine the position of its edge E.

The amounts of light received by the photoreceptor elements 20 and thecorresponding output voltages thereof may differ depending on differencein light reflectivity of the sheet P. Accordingly, the output voltagesfrom the photoreceptor elements 20 may differ, as illustrated in FIG. 6.

For example, the output voltages from the photoreceptor group 20 e isgreater than the threshold Fth when the sheet is the light-coloredpaper, as illustrated in FIG. 6. Similarly, the output voltage from thephotoreceptor group 20 f is greater than the threshold Fth when thesheet is the dark-colored paper. The above problem may be solved byadjusting the amount of light emission.

Next, the adjustment of the amount of light emission is described. Thelight emission element 17 of the sheet detector 14 or 14 a is capable ofadjusting the amount of light emission. The type of sheet may be inputfrom the operation part 25 illustrated in FIG. 1. The controller 40 mayreceive the input of the type of sheet and serves as a detectioncondition setter to set detection condition of the sheet detector 14.

FIG. 7 illustrates an example of the operation part 25 when thetransmission sheet detector 14 illustrated in FIG. 2 is used. Theoperation part 25 may include a numeric keypad and a display 26. Thedisplay 26 may include a sheet selection part 30 including a normalpaper key 27, a thin paper key 28, and a cardboard key 29, an OK key 31,and/or a cancel key 32. The sheet selection part 30 may serve as a sheettype setter according to example embodiments.

When a normal paper (normal thickness) is contained in the sheetcassette 23, an operator pushes the normal key 27 and the OK key 31. Theoperator may push the cancel key 32 and repeat the selection when awrong key is pushed. An input signal to set the detection condition maybe sent to the controller 40. The controller 40 may send a command toset the amount of light emission to normal. The light emission element17 may emit normal amount of light when the sheet P being a normal paperis conveyed to the sheet detector 14 when detection condition is set asabove. Therefore, the sheet detector 14 may detect the position of thesheet P in the width direction by determining that the sheet P existsthe position facing the photoreceptor element 20 in the photoreceptorelement group 20 a whose output voltage is equal to or less than thethreshold Fth.

When a thin paper is contained in the sheet cassette 23, the operatormay push the thin key paper 28 and the OK key 31. An input signal to setthe detection condition may be sent to the controller 40. The controller40 may send a command to decrease the amount of light emission.

When a cardboard (thick paper) is contained in the sheet cassette 23,the operator pushes the cardboard key 29 and the OK key 31. An inputsignal to set the detection condition may be sent to the controller 40.The controller 40 may send a command to increase the amount of lightemission.

The light emission element 17 may adjust the amount of light to detectthe position of the sheet P according to the type of sheet as describedabove. Therefore, the sheet detector 14 may more accurately detect theposition of sheet P in the width direction based on the same thresholdFth, whichever the sheet P is a thin paper, a normal paper, or acardboard. The amount of light emission may be adjusted so that the eachphotoreceptor element 20 facing the sheet P outputs the value equal toor less than the threshold value, whichever the type of sheet is.

FIG. 8 illustrates another example of the operation part 25 when thereflection sheet detector 14 a illustrated in FIG. 4 is used. The sheetselection part 30 a may include a normal paper key 27 a, a light-coloredpaper key 28 a, and/or a dark-colored paper key 29 a.

The operator may push the normal paper key 27 a when the sheet P is anormal paper having a normal reflectivity, the light-colored paper key28 a when the sheet P is a light-colored paper having a higherreflectivity, or the dark-colored paper key 29 a when the sheet P is adark-colored paper having a lower reflectivity. The operator may pushthe OK key 31. The light emission element 17 may be set to emit a normalamount of light when the sheet P is a normal paper, a decreased amountof light when the sheet P is a light-colored paper, or an increasedamount of light when the sheet P is a dark-colored paper.

The sheet detector 14 a may detect the position of the sheet P in thewidth direction after the detection condition is set as above.Therefore, the sheet detector 14 a may correctly detect the position ofsheet P in the width direction based on the same threshold Fth,whichever the sheet P is a normal paper, a light-colored paper, or adark-colored paper. The amount of light emission may be adjusted so thatthe each photoreceptor element 20 facing the sheet P outputs the valueequal to or greater than the threshold value, whichever the type ofsheet is.

Whichever the sheet detector according to example embodiments is areflective detector or a transmission detector, the detection conditionaccording to characteristics of sheet (e.g. thickness and reflectivity)may be set by a detection condition setter to adjust the amount of lightemission before the position detection. Therefore, the sheet detector 14according to example embodiments may detect the position of sheets inits width direction based on the same threshold even if the sheets havedifferences in characteristics.

In example embodiments, an operator may push a selection key to inputthe type of sheet. Alternatively, the sheet detector 14 according toexample embodiments may detect the type of sheet. For example, the lightemission element 17 may emit light to the sheet P. The photoreceptorelement 20 g may receive light penetrating the sheet P in the case ofthe transmission detector, or light reflected by the sheet P in the caseof the reflection detector. The type of sheet (characteristics) may bedetermined according to the output voltage from the photoreceptorelement 20 g. Alternatively, a detecting device to determine the type ofsheet may be provided upstream of the sheet detector 14 in the sheetconveyance direction in FIG. 1.

Next, examples of detection condition setting procedures performed bythe sheet detector 14 are described with reference to flowcharts ofFIGS. 9 and 10.

Referring to FIG. 9, when a sheet P is sent from the sheet feeder 21illustrated in FIG. 1 and reaches the sheet detector 14 (S1), thedetection condition setting procedure is started (S2). The lightemission part 15 may emit a first light to the sheet P. The amount ofthe first light is defined as Q_(a) (S3). The photoreceptor elements 20may receive light penetrating the sheet P. The output voltage from thephotoreceptor element 20 facing the sheet P, for example, thephotoreceptor element 20 g, may be defined as X_(a). A range of outputvoltage, Z_(a) plus or minus z_(a) (Z_(a)±z_(a)), is predetermined todetermine whether or not the type of sheet P is type T_(a). Thecontroller 40 may determine whether or not the output voltage X_(a) iswithin the range of Z_(a)±z_(a) (S4). When the output voltage X_(a) iswithin the range of Z_(a)±z_(a), the sheet P may be judged as the typeT_(a). Accordingly, the threshold Fth may be set to F_(a) (S5). Thedetection condition setting procedure may be completed (S6).

The position of sheet P in its width direction may be detected based onthe amount of the first light Q_(a) from the light emission part 15 andthe threshold F_(a) that are set as above.

When the output voltage X_(a) is not within the range of Z_(a) ±z_(a),the light emission part 15 may emit a second light whose amount isdifferent from the amount of the first light Q_(a) (S7). The amount ofthe second light may be defined as Q_(b). The output voltage from thephotoreceptor element 20 g may be defined as X_(b). A range of outputvoltage Z_(b)±z_(b) is predetermined to judge whether or not the type ofsheet P is type T_(b). The controller 40 may determine whether or notthe output voltage X_(b) is within the range of Z_(b)±z_(b) (S8). Whenthe output voltage X_(a) is within the range of Z_(b)±z_(b), the sheet Pmay be judged as the type T_(b). The threshold Fth may be set to F_(b)(S9). The detection condition setting procedure is completed (S10).

The position of sheet P in its width direction may be detected based onthe amount of the second light Q_(b) from the light emission part 15 andthe threshold F_(b) that are set as above.

Further, when the output voltage X_(b) is not within the range ofZ_(b)±z_(b), the light emission part 15 may emit a third light whoseamount is different from the first light Q_(a) or second light Q_(b)(S11). The output voltage from the photoreceptor element 20 g may bedefined as X_(c). A range of output voltage Z_(c)±z_(c) is predeterminedto judge whether or not the type of sheet P is type T_(c). Thecontroller 40 may determine whether or not the output voltage X_(c) iswithin the range of Z_(c)±z_(c) (S12). When the output voltage X_(c) iswithin the range of Z_(c)±z_(c), the sheet P may be judged as the typeT_(c) and the threshold Fth is set to F_(c) (S13). The detectioncondition setting procedure is completed (S14).

The position of sheet P in its width direction may be detected based onthe amount of light Q_(c) from the light emission part 15 and thethreshold F_(c) that are set in the above setting procedure. The aboveprocedure may be continued and similar processes may be performed asrequired (S15). The position of sheet P in its width direction may bedetected based on another amount of light from the light emission part15 and another threshold that are set in the continued settingprocedure.

Processes performed by the sheet detector 14 a may be substantially sameas in the flowchart of FIG. 9, except that the photoreceptor elements 20may receive the light reflected by the sheet P instead of the lightpenetrating the sheet P.

The judgment of the type of sheet is explained more specifically, takingan example in which the amounts of the first, second, and third lightsQ_(a), Q_(b), and Q_(c) satisfy the relation of Q_(a)<Q_(b)<Q_(c). Whenthe output voltage X_(a) is within the range of Z_(a)±z_(a) at S4, thetype T_(a) is determined as a thin paper at S5. In the case of the sheetdetector 14 a, the type T_(a) is determined as a light-colored paper.Accordingly, the threshold F_(a) to detect the position of the sheet Pas a thin paper or a light-colored paper may be set.

Alternatively, when the output voltage X_(b) is within the range ofZ_(b)±z_(b) at S8, the type T_(b) is determined as normal paper having anormal thickness or normal reflectivity at S9. Accordingly, thethreshold F_(b) to detect the position of the sheet P as a normal papermay be set.

Alternatively, when the output voltage X_(c) is within the range ofZ_(c)±z_(c) at S12 in FIG. 9, the type T_(c) is determined as cardboardor dark-colored paper at S13. Accordingly, the threshold F_(c) to detectthe position of the sheet P as a normal paper may be set.

As described above, the controller 40 as the detection condition setterin the above example may cause the light emission part 15 to emit thefirst, second, and third lights whose amounts are different from eachother. The controller 40 may determine whether or not the output voltageof the photoreceptor element 20 g is within the range of Z_(a)±z_(a),Z_(b)±z_(b), or Z_(c)±z_(c). The controller 40 may judge the type ofsheet P based of the determination. The controller 40 may be configuredto set the amount of light emission Q_(a), Q_(b), or Q_(c) and thethreshold F_(a), F_(b), or F_(c), corresponding to the type of sheetjudged as above before the detection of the position of the sheet P.

The amounts of light emission Q_(a), Q_(b), and Q_(c) and the thresholdsF_(a), F_(b), and F_(c) may be held in combination. When a sheet Preaches the sheet detector 14, the light emission element 15 may emitevery amount of light held and measure corresponding output voltages.The controller 40 may select a combination of amount of light emissionand threshold to detect the position of sheet based on the outputvoltages. A plurality of amounts of light emissions and thresholds maybe held as default values corresponding to typical types of sheetincluding a normal paper, a cardboard, a thin paper, and asemitransparent paper. The amount of light emission and threshold may beappropriately set corresponding to the type of sheet.

In the case of the detection condition setting procedure in FIG. 9, thecontroller 40 may determine whether or not an output voltage of thephotoreceptor element 20 g is within a predetermined range. Thecontroller 40 may judge the type of sheet based on the range and setsthe amount of light emission and the threshold to the valuescorresponding the type of sheet. Therefore, the threshold value may notbe quite accurate.

For example, when the amount of light emission is Q_(a) at S3 in FIG. 9,appropriate thresholds may be different in both cases in which theoutput voltages are Z_(a)+z_(a) and Z_(a)−z_(a). However, the detectioncondition setter in FIG. 9 may set the threshold to F_(a), regarding theboth output voltage as Z_(a). Thus, an accurate threshold may not beselected when the output voltage slightly varies from Z_(a), which maydecrease the accuracy of the position detection of sheets.

Therefore, another example of detection condition setting procedure tocope with the above problem is described with reference to the flowchartof FIG. 10. In FIG. 10, processes similar to the processes in FIG. 9 areperformed, except for S105-1, S109-1, and S113-1.

The photoreceptor element 20 g may output an output voltage X_(a), whenthe first light whose amount is Q_(a) is emitted at S103. When thecontroller 40 determines the output voltage X_(a) is within the outputvoltage range of Z_(a)±z_(a) at S104, the sheet P may be determined tobe type T_(a) and the threshold is set to F_(a) at S105. When the actualoutput voltage X_(a) varies from the center value Z_(a) of the range ofZ_(a)±z_(a), the controller 40 may adjust the amount of light emissionaccording to the variation at S105-1. The position of the sheet P may bemore accurately detected based on the threshold F_(a).

Likewise, when the actual output voltages X_(b) and X_(c) varies fromthe center values Z_(b) and Z_(c) of the predetermined or desired outputvoltage ranges, the controller 40 may adjust the amounts of lightemission according to the variations at S109-1 and S113-1, respectively.

The output voltage of the photoreceptor element 20 g may be approximatedto the target values by adjusting the amounts of the first, second, andthird lights Q_(a), Q_(b), and Q_(c) as described above. Therefore, theposition of the sheet P may be more accurately detected with theselected threshold F_(a), F_(b), or F_(c). For example, to adjust theamount of first light Q_(a), an adjustment value q_(a) is added to anddeducted form Q_(a) and the output voltage is measured. Until the outputvoltage that is closest to the Z_(a) is obtained, different adjustmentvalues are added to and deducted from the Q_(a). As a result, the amountof light emission and the output voltage threshold may be set to moreappropriate values corresponding to various types of sheets.

An assumption to perform the procedure example of FIG. 10, the amount oflight emission may be adjusted according to the difference between X andZ, when the following relation is satisfied:(Z+ΔZ) >=X=>(Z−ΔZ)

where a predetermined or desirable output voltage range is Z +ΔZ, thecenter value is Z, the allowance in the range is ΔZ, and the actualoutput voltage is X.

The sheet P may be stopped when its front edge is sandwiched between theregistration rollers 11 and kept motionless. In this condition, thedetection condition setting procedure in FIGS. 9 or 10 may be performed.In the above situation, the detection conditions may be properly seteven if the sheet transport speed is high and/or the sheet P is short inthe sheet conveyance direction. Alternatively, the sheet P may bestopped by another member for the sheet detector to perform positiondetection of the sheet P.

The detection condition setting procedure may be performed for everysheet transported from the sheet feeder 21 illustrated in FIG. 1.However, a complicated control may be required in the above case.Because one type of sheets are generally contained in a sheet feeder 21,the sheet detector 14 may be configured to perform the detectioncondition setting procedure for a first sheet transported from the sheetfeeder 21. The positions of following sheets may be detected based onthe detection conditions set relative to the first sheet. Therefore, thecomplicated control may be unnecessary.

Alternatively, the sheet detector 14 may be configured to perform thedetection condition setting procedure for a first sheet transported fromthe sheet feeder 21 after the image forming apparatus 100 illustrated inFIG. 1 is powered on.

In the case of the sheet conveyance unit 10 having a sheet set detector,the sheet detector 14 may be configured to perform the detectioncondition setting procedure for a first sheet transported from the sheetfeeder 21 after the sheet set detector detects the sheet. In this case,the complicated control may be similarly unnecessary.

The sheet selection part 30 illustrated in FIGS. 7 and 8 are examples ofthe sheet type setter. In the case of the image forming apparatus havingthe sheet type setter, the sheet detector 14 may be configured toperform the detection condition setting procedure for a first sheettransported from the sheet feeder 21 after the type of sheet is changedwith the sheet type setter.

Numerous additional modifications and variations are possible in lightof the above teachings. It is therefore to be understood that within thescope of the appended claims, the disclosure of this patentspecification may be practiced otherwise than as specifically describedherein.

This patent specification is based on Japanese patent applications, No.JP2005-368849 filed on Dec. 21, 2005 in the Japan Patent Office, theentire contents of each of which are incorporated by reference herein.

1. A sheet detector mechanism comprising: a sheet detector including, alight source emitting light, and a plurality of photoreceptors arrangedin a line in a main-scanning direction spaced at substantially equalintervals to extend across a sheet skew marginal width and to evenlyreceive light from a light source, the plurality of photoreceptorsincluding a first photoreceptor group overridable by a side edge of thesheet passing through a sheet conveyance passage and a secondphotoreceptor group connected to the first photoreceptor group andincluding an opposite end of the plurality of photoreceptors being outof the side edge of the sheet, the first photoreceptor group changingoutput voltages when overridden by the side edge of the sheet; and acontroller configured to adjust a light emission amount of the lightsource in accordance with characteristics of the sheet so as to maintainthe output voltages of the plurality of photoreceptors, regardless ofthe characteristics of the sheet.
 2. The sheet detector mechanism ofclaim 1, wherein the light source and the plurality of photoreceptorsform a transmissive type sheet detector, and the controller is furtherconfigured to previously store a threshold value and to determine theoutput voltages of the plurality of photoreceptors as detecting thesheet when the output voltages are equal to or smaller than thethreshold value.
 3. The sheet detector mechanism of claim 1, wherein thelight source and the plurality of photoreceptors form a reflection typesheet detector, and the controller is further configured to previouslystore a threshold value and to determine the output voltages of theplurality of photoreceptors as detecting the sheet when the outputvoltages are equal to or greater than the threshold value.
 4. The sheetdetector mechanism of claim 1, wherein the controller configured toadjust a light emission amount of the light source in accordance withthe characteristics of the sheet before sheet detection by the sheetdetector so as to maintain the output voltages of the plurality ofphotoreceptors, regardless of the characteristics of the sheet.
 5. Thesheet detector mechanism according to claim 1, wherein the sheetdetector performs a detection setting procedure while the sheet is keptmotionless.
 6. The sheet detector mechanism of claim 1, wherein thelight source faces a first surface of the sheet and the photoreceptorpart faces a second surface of the sheet when the sheet is conveyed tothe sheet detector.
 7. The sheet detector mechanism of claim 1, whereinthe light source and the plurality of photoreceptors face a same surfaceof the sheet when the sheet is conveyed to the sheet detector.
 8. Animage forming apparatus, comprising: a sheet conveyance mechanismconfigured to convey a sheet through a sheet conveyance passage; animage forming mechanism configured to form an image and to transfer theimage at an image transfer region onto the sheet conveyed by the sheetconveyance mechanism; and the sheet detector mechanism of claim 1,arranged upstream from the image transfer region in the sheet conveyancedirection.
 9. The image forming apparatus according to claim 8, whereinthe sheet conveyance mechanism further comprises: a sheet type setter toset a type of sheet; and the sheet detector performs a detection settingprocedure for a first sheet conveyed from the sheet conveyance mechanismafter the sheet type is changed by the sheet type setter.
 10. The imageforming apparatus according to claim 8, wherein the sheet detectorperforms a detection setting procedure for a first sheet conveyed fromthe sheet conveyance mechanism.
 11. The image forming apparatusaccording to claim 8, wherein the sheet detector performs a detectionsetting procedure for a first sheet conveyed from the sheet conveyancemechanism after the image forming apparatus is powered on.
 12. The imageforming apparatus according to claim 8, configured to be a multifunctionperipheral including at least two functions selected from copying,printing, and facsimile functions.
 13. The image forming apparatus ofclaim 8, wherein the controller is further configured to perform a sheetanalysis test in which the controller controls the light source tosequentially emit the light by changing the light emission amountrelative to the sheet passing through the sheet conveyance passage anddetermines the characteristics of the sheet based on the output voltageswith respect to the changes of the light emission amount, and to adjustthe light emission amount of the light source in accordance with aresult of the sheet analysis test before a performance of a sheetdetection by the sheet detector so as to maintain the output voltages ofthe plurality of photoreceptors, regardless of the characteristics ofthe sheet.
 14. The image forming apparatus of claim 13, wherein thecontroller determines the characteristics of the sheet based on theoutput voltages with respect to the changes of the light emission amountby examining a relationship (Z+ΔZ)≧X≧(Z−ΔZ), where Z is a center valueof a predetermined expected output voltage range, ΔZ is an allowancevalue, and X is an actual output voltage, wherein the controller adjuststhe light emission amount of the light source in accordance with adifference between X and Z.